1. Field
The disclosure relates to an optical transmitting apparatus that modulates light and a control method of the optical transmitting apparatus.
2. Description of the Related Art
Toward a higher capacity and a longer distance of an optical transmission system, evaluation is being made of various modulation systems for transmission of an optical signal. Research and development are being made of the optical transmission systems using various phase modulation systems such as a CSRZ (Carrier Suppressed RZ) modulation system, an optical duobinary modulation system, a DPSK (Differential Phase Shift Keying) system, a SSB (Single Side Band) modulation system, and a DQPSK (Differential Quadrature PSK) system as well as an NRZ (Non Return to Zero) modulation system and an RZ. (Return to Zero) modulation system that are so far applied to products.
In a transmitting unit of the optical transmission systems using these modulation systems, a stabilizing technology for constituent components such as a modulator is essential to stabilization of an optical transmission signal. For example, an ABC (Auto Bias Control) circuit for preventing deterioration of a transmission signal due to a drift of an LN modulator in the NRZ modulation system may be cited as such a technology.
A multilevel phase modulation system such as the SSB modulation system and the DQPSK system is required to perform a phase shift control that is not required for the NRZ modulation system. Specifically, by shifting a relative phase of respective lights of an I arm and a Q arm of a modulating unit, a phase difference between the respective lights must be adjusted to nπ/2 (n is an arbitrary odd number) (see, e.g., Japanese Patent Application Laid-Open Publication No. 2007-82094). For example, a low-frequency signal is superimposed over a control signal supplied to a phase shift unit and synchronous detection is performed based on a low-frequency component of an output light of an optical transmitting apparatus.
However, the above conventional technology has problems of a low sensitivity of a monitoring signal and lowering of control stability, depending on characteristics of constituent components such as an output power of an LD, a loss of the modulator, and a light-receiving sensitivity of a monitoring photodetector.
The above conventional technology causes variations in rising and falling characteristics of a data signal, depending on a driver that inputs the data signal for modulation to a light modulator. If the rising and the falling of the data signal are not steep, there is a problem that control accuracy is decreased due to lowering of the monitoring signal sensitivity to a deviation of a phase difference between arms.